High breast density is associated with increased risk of breast cancer. This epidemiological study will test the hypothesis that in-utero exposure to environmental endocrine-active compounds (organochlorine compounds, p,p'-DDT, p,p'-DDE, o,p'-DDT and individual PCB congeners) alters breast density in women measured at 40-44 years of age. The study population (N=601) is a subset of the Child Health and Development Studies (CHDS), a California cohort that has been followed for more than 40 years, beginning before birth. We have already shown that in-utero exposures to organochlorine compounds are 1) high and variable in the CHDS cohort and 2) are associated with reproduction (time to pregnancy) in adult daughters. The proposed study will use existing data on breast density and health status in CHDS daughters at age 40-44 years of age, which are currently being collected in two NIH funded projects (P01 AG023028-01, Ezra Susser, PI and RO1 CA104842-01, Mary Beth Terry, PI). The proposed project would fund assays of maternal prenatal serum for organochlorines and related new data analysis. In addition, existing data in the CHDS provide an opportunity to examine important confounding or mediating factors, which include 1) Prospective data on maternal estrogen and androgen levels during the daughter's gestation, maternal reproductive and menstrual history, maternal body mass, and pregnancy weight gain, maternal smoking and alcohol use, whether daughter was breastfed, daughter's growth in-utero, infancy, early and and middle childhood;and 2) Retrospective and cross-sectional data on daughter's menstrual and reproductive history, adult body size, health and health behaviors. This would be the first longitudinal study of human breast density that can measure exposure to organochlorines in-utero, when the fetus may be highly vulnerable. Findings will be relevant to understanding and predicting the breast cancer risk of women born in the 1960's, who were exposed to high levels of DDT in-utero. These women have not yet achieved the age of high breast cancer risk. The proposed project is an efficient use of existing resources for a unique 40-year follow-up study.